This is a renewal application for a grant to study the mechanism of action of the 44-amino acid bovine papillomavirus E5 oncoprotein. This grant supported the discovery that the E5 protein transforms cells by specifically binding the transmembrane (TM) domain PDGF receptor (PDGFR), resulting in receptor dimerization and activation. The E5 protein is essentially a free-standing TM domain. Our studies of the E5 protein have provided novel insights into protein-protein interactions occurring within membranes, a large class of important interactions, which have been difficult to study. In the course of this work, we developed genetic techniques to construct and isolate artificial small proteins modeled on the viral E5 protein, which we have named traptamers, including the simplest oncoproteins ever reported. These experiments demonstrated that the sequence of the membrane-spanning segment is sufficient to specify the identity of the target protein. Here, we will conduct genetic, biochemical and computational studies to refine and test the model of the complex between the TM domain of the PDGFR and the BPV E5 protein. This will include experiments designed to identify the TM domain dimer interface of the PDGFR activated by the E5 protein and to test specific predictions made by the model. Next, we will determine the basis for signaling differences induced by the BPV E5 protein and related small TM oncoproteins, with a focus on testing the hypothesis that different small TM proteins alter the orientation of the PDGFR TM domains and differentially affect the sites of tyrosine phosphorylation. We will also determine the basis for the activity of inhibitory small TM proteins we have already isolated and synthesize and study biologically active peptides based on small TM proteins. Finally, we will characterize TM oncoproteins with minimal chemical complexity, consisting of certain sequences of only leucine and isoleucine. We will identify the sequence features of these ultrasimple oncoproteins responsible for activity and we will determine their specificity. These experiments will provide new insights into the mechanism of action of an unusual viral oncoprotein and artificial proteins modeled on it. More generally, in the tradition of using viral proteins as probes for important biochemical activities and cellular processes, these experiments will help define the principles that govern interactions between proteins in cell membranes, interactions that are important in the genesis of human cancer.